Shoulder Soreness/ Partial Mastectomy

10 years ago I had a partial mastectomy and had never really noticed the extreme difference in my breasts until after being diagnosed with secondary breast cancer in my hips and spine.  Since about mid this year I am having a lot of problems with pain in my left shoulder coming down the front and back a short ways and down my arm.  Ive noticed now my right breast, the one that had cancer, is now very much smaller than my left and was wondering if the pain may have something to do with the larger one pulling to the left side all the time.  I was told only yesterday you can buy an insert to put in your bra to even up but don`t know if thats the answer or even if its to do with the meds I`m now on.